1. Field of the Invention
The present invention relates to a wideband transformer/balun and more particularly, to a miniature wideband transformer/balun having a bandwidth of multiple decades. Such transformers find application in high frequency mixer circuits.
2. Description of the Prior Art
This invention generally relates to transformer/baluns surface mounted on printed circuit boards and more particularly to baluns for use with high frequency mixer circuits for converting balanced ports to unbalanced ports. The word balun comes from combining the words “BALanced” and “UNbalanced”. A typical high frequency mixer circuit may be realized as a Monolithic Microwave Integrated Circuit, (MMIC) which is a type of integrated circuit (IC) device that operates at microwave frequencies (300 MHz to 300 GHz).
In a typical high frequency mixer application, each of the RF and LO differential ports should ideally be driven through a balun for optimal performance. Some MMIC mixers have an operation range of 10 MHz to 6 GHz. Lacking a single broadband balun, as many as three baluns would be needed to cover this frequency range, with each having an individual mechanical footprint and therefore each needing an individual pc board layout. This becomes unacceptable for miniaturized wideband applications.
In general, industry uses LTCC or semiconductor technology for high frequency designs, (above 0.5 GHz) but these technologies have limitations. A first limitation is that the low frequency performance is constrained by the physical size of the device. A second limitation is that the bandwidth of an LTCC or semiconductor design cannot currently be multi-decade; it is at best multi-octave.
U.S. Pat. No. 5,523,728 entitled MICROSTRIP DC TO GHZ FIELD STACKING BALUN teaches a pc board based balun that maintains low insertion loss and good balance for ultra wide band (DC-to-Ghz) applications. These results are achieved using a balun structure formed by microstrip lines on a dielectric substrate. While the claimed bandwidth is considerable, the upper frequency limit is not explicitly stated. The lower frequency operation also requires the use of multiple ferrite cores. Further, the balun is relatively large and either occupies a dedicated area on a pc board or is fabricated as a discrete pc board which must then be handled and assembled into a still larger circuit.
U.S. Pat. No. 5,379,006 entitled WIDEBAND (DC TO GHZ) BALUN teaches a balun that maintains low insertion loss and good balance for ultra wide band applications. While the claimed bandwidth is considerable, the upper frequency limit is not explicitly stated. The lower frequency operation also requires the use of multiple ferrite cores. This balun also requires relatively long transmission lines to be fabricated, and in the forms disclosed will require a complex mounting or support structure.
U.S. Pat. No. 7,205,861 entitled BALUN DEVICE teaches a wideband balun fabricated from tuned LC networks. However, the stated bandwidth for this balun is only 3 GHz to 10 GHz, not even a full decade and completely lacking in the lower frequency range.
U.S. Pat. No. 7,250,828 entitled COMPACT BALUN teaches a distributed backwards-wave balun comprising pairs of coupled transmission lines. However, this patent fails to disclose either the resulting dimensions or the usable frequency range for the balun.
U.S. Pat. No. 6,531,943 entitled BALUN-TRANSFORMER teaches a balun fabricated from a chemically etched conductor-backed coplanar waveguide connected to a conductor-backed coplanar stripline, and including an impedance matching circuit. The claimed bandwidth is from several MHz to 10 GHz, but the structure requires highly specialized processing and is potentially unacceptably large.
U.S. Pat. No. 6,278,340 entitled MINIATURIZED BROADBAND BALUN TRANSFORMER HAVING BROADSIDE COUPLED LINES teaches a miniaturized wideband multi-layer balun. This balun is fabricated from multiple layers of metalized dielectric substrates, and is only analyzed over a frequency range of 1.6 GHz to 2.4 GHz.
U.S. Pat. No. 8,228,154 entitled MINIATURIZED WIDE-BAND BALUNS FOR RF APPLICATIONS teaches a balun having a bandwidth of 800 MHz to 6 GHz, designed to be fabricated using semiconductor technologies. While this balun offers the advantage of being potentially integrated directly into a larger integrated circuit, it is lacking adequate lower frequency response and requires all the complexity of a semiconductor fabrication line in order to be made.
A paper entitled Decade Bandwidth Planar MMIC Balun published in the Microwave Symposium Digest in 2006 teaches a balun operating over a 2 GHz to 20 GHz frequency range by connecting low and high band baluns to three diplexer filters which are used as input and output conditioning devices. While small in size, this balun fails to operate at lower frequencies.
Many of the prior art baluns discussed herein use ferrite cores, which are generally round. Conventional binocular ferrite cores have round edges and round apertures. When magnet wire is wound with spacing between turns on these rounded shapes it will tend to move during and after winding, thereby reducing the coil precision and inducing electrical parameter variation.
A more capable balun would provide a frequency range of 10 MHz to 6,000 MHz or higher, with a substantially flat insertion loss and a low return loss with minimal parameter variation between units while having a highly miniaturized form factor, be fabricated out of readily available materials, and be surface mountable using standard robotic pick and place machinery.